Corrosion control of electrical cables used in cathodic protection

ABSTRACT

This invention resides in corrosion control of electrical cables associated with cathodic protection systems. In the preferred embodiment the cable is in the form of passive metals chosen for electrolysis-corrosion-resistance, including titanium, tantalum, niobium, zirconium, nickel, or alloys thereof. Small diameters may be used to make the cable flexible. The wires may be bare or enclosed insulator materials for easy handling. The invention which is applicable to the “impressed current” and “sacrificial anode” methods, is particularly advantageous in situations where the cable/structure is embedded deep in soil or concrete, and cannot be easily repaired or replaced if any damage occurs.

FIELD OF THE INVENTION

-   -   This invention relates generally to corrosion control of the         electrical cables associated with cathodic protection system         used in various electrolytes, including soil, water, seawater,         and concrete.

BACKGROUND OF THE INVENTION

Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with a piece of another more easily corroded “sacrificial metal” to act as the anode of the electrochemical cell. The sacrificial metal then corrodes instead of the protected metal. For structures where passive galvanic CP is not adequate, for example in long pipelines or in sea water, an external DC electrical power source is sometimes used to provide current (the “impressed current” method).

Cathodic protection systems are used to protect a wide range of metallic structures in various environments. Common applications are; steel water or fuel pipelines and storage tanks such as home water heaters, steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust.

Electrical copper cables used for cathodic protection systems which are immersed in water, buried in soil, or embedded in concrete, are designed to be environmentally resistant. Insulation materials commonly used to protect the copper conductors from corrosion include HMWPE, RHH-USE, XLPE/PVC, PVDF/HMWPE, and Kyner/HMWPE. In general, using such materials, typical cable life expectancy in an aggressive or corrosive environment might be 30 years.

Over time, however, the copper electrical cables connecting with the anodes used for cathodic protection systems are subject to corrosion. Even though the anode itself is designed for long life, the copper conductors connecting to the anode often fail due to the electrolysis corrosion caused by the damage to insulation, intrusion of corrosive liquids or gases under the insulation, or the end of the insulation material life. Many copper anode cables fail pre-maturely due to one or more of the following factors:

1. Small cracking in the insulator caused by coiling process,

2. Physical damage during installation,

3. Penetration of chlorine gas evolution on the anode surface into the cable,

4. Development of microchannel voids between the insulator and copper conductors,

5. Chemical attack by hydrochloric acid caused by chlorine evolution produced by the anode,

6. Improper workmanship on the cable connections,

7. Poor durability of the cable splicing material and method,

8. Wrong insulator type for a particular environment, and

9. Free-thaw damage to the insulator located near the ground surface.

In addition, when the cable is installed in dry soil or concrete, the heat generated by the cathodic protection current does not dissipate due to the low thermal conductivity. As a result, the cable temperature increases with time, causing premature failure.

When a copper conductor discharges direct current to the surrounding electrolyte, the copper is consumed in a short period of time due to electrolysis corrosion. If chlorine gas, which is produced by cathodic protection anodes in the electrolyte containing chlorides, intrudes into the cable from any damaged area or poorly insulated cable splice, it travels though under the insulator of among the copper wires. As a result, the copper conductor is quickly damaged and pre-mature failure of the copper cables often occurs. When the cables fail in deep soil or embedded concrete, it is often not possible to replace or repair the damaged cables.

Ironically, anode life has been increasing significantly due to the new anode technologies. One hundred years of design life for cathodic protection anode systems has become common for new concrete structures. In such situations, failure of the copper cabling is incompatible with such long-life systems.

SUMMARY OF THE INVENTION

This invention improves upon existing cathodic protection systems by providing a passive metal electrical interconnection. The passive metal interconnection may include titanium, tantalum, niobium, zirconium, nickel, or alloys thereof. The passive metal passive metal electrical interconnection may be coated with a mixed-metal oxide (MMO). To provide flexibility, the passive metal passive metal electrical interconnection may include one or more wires, each with a diameter of 3 mm or less. In particular, the interconnection may include a plurality of wires, each with a diameter of 1 mm or less. Alternatively, the interconnection may include a plurality of thin strips, each having a thickness in the range of 0.1-1 mm.

The passive metal passive metal electrical interconnection may be comprised of bare (exposed) metal wires, or the wire or wires may be insulated with existing of yet-to-be-developed jackets. As a further option, the electrical interconnection may comprise one or more passive-metal-clad copper wires.

The invention is applicable to the “impressed current” protection method, as well as the “sacrificial anode” method, particularly if the target anode design life is very long (i.e., decades). While the invention is typically used for anode connections, the technique may also be applied to cathode connections and structures, especially when long-life cathodic protection is required. The invention is particularly advantageous in situations where the cable/structure is embedded deep in soil or concrete, and cannot be easily repaired or replaced if any damage occurs.

DETAILED DESCRIPTION OF THE INVENTION

This invention improves upon the existing art by using passive metal conductor cables in direct-current cathodic protection systems to prevent corrosion, even when the insulator fails. The invention takes advantage of the fact that passive metals such as titanium, tantalum, niobium, zirconium, nickel, any alloys thereof, exhibit a significantly higher resistance to electrolysis corrosion below the passive breakdown potential. In other words, when the conductors of the cables used in cathodic protection systems are made with passive metals, the failure of the insulator in electrolyte does not cause corrosion as long as it is operated below the passive film break-down potential.

When an anode cable is made with bare titanium conductor wires, for example, it does not corrode due to failure of the insulator for any reason. Since the titanium passive film exposed to any electrolyte has the property of high resistance to the electrolyte, it prevents discharge of the current it conducts.

While titanium is the preferred passive metal for lower-voltage protection systems, when high voltage operation is required, other metal such as niobium or tantalum can be used. Typical breakdown potentials of niobium or tantalum are 40 volts and 80 volts, respectively, in chloride free electrolyte. Another option is to coat the titanium wires with mixed-metal oxides when high-voltage operation is required. MMO coated anodes are manufactured by coating a mixture of precious metal oxides on a specially treated precious metal. A useful review of MMO-coated anodes and installation techniques may be found in “Cathodic Protection of Steel in Concrete” By Paul Chess, Taylor & Francis (1998), ISBN 0419230106, the entire content of which is incorporated herein by reference.

When titanium is coated with mixed metal oxides (MMO), the breaking potential significantly increases. The coating thickness of MMO coating can be in the range of 1 micron to 30 microns depending on the design life. Cu coated with titanium is typically called clad wire. To avoid any exposure of copper, which may represent a serious problem, the coating may be on the order of 50 to 100 microns. However, since a MMO-coated titanium cable may discharge current to the surrounding electrolyte if the insulator fails, this should be considered in cathodic protection system design.

One of the problems in using passive metals such as titanium for cabling is breakage due to the brittle nature of the material. However, it has been discovered that by making the cables with small diameter wires or thin strips, the cable is flexible enough to coil the cable for easy handling and shipping. In accordance with the invention, the wire diameter can be in the range from 0.1 to 3 mm, but more preferably in the range of 0.3 mm to 1 mm. As such, the number of conductors may be 3000 for 0.1 mm wires, or a single wire at 3 mm. More preferably, however, 20 to 100 wires at a 0.5 mm diameter, or 10 to 50 wires at 0.8 mm are used. While in some embodiments an outside jacket may not even be required, the wires are preferably encased in a conventional cable insulator for easy anode assembling and installation.

Another consideration is that the electrical resistivity of passive metal conductors is much higher than that of copper. This can be overcome by using a greater number of conductor wires in the cable based on the required conductance. While the acceptable resistance depends upon the system design and application, perhaps a maximum resistance in in the range of 5-50 ohms, more preferably on the order of 10 ohms, with the diameter and number of wires being selected to meet this target range. Another option is to coat the copper wires with titanium or other passive metal(s) to achieve the combined benefit of higher conductivity and corrosion resistance.

The present invention comprises the following simultaneous advantages and benefits:

-   -   With cathodic protection cables utilizing passive metal         conductors, any damage to the insulator does not cause the         failure of the cathodic protection system;     -   When the insulator reaches its end of the life, the passive film         on the conductor wires continues to prevent discharge of the         cathodic protection current to the electrolyte, enabling the         anode system itself to reach its maximum life potential.     -   The resistance of the passive metal conductor cable can be         adjusted by increasing the number of the conductors.     -   The flexibility of the passive metal conductor cable can be         achieved by using smaller-diameter wires. 

I claim:
 1. In a cathodic protection system having an anode, the improvement comprising: a passive metal electrical interconnection to the anode.
 2. The improvement of claim 1, wherein the passive metal includes titanium, tantalum, niobium, zirconium, nickel, or alloys thereof.
 3. The improvement of claim 1, wherein the passive metal passive metal electrical interconnection is coated with a mixed-metal oxide.
 4. The improvement of claim 1, wherein the passive metal passive metal electrical interconnection includes one or more wires, each with a diameter of 3 mm or less.
 5. The improvement of claim 1, wherein the passive metal passive metal electrical interconnection includes a plurality of wires, each with a diameter of 1 mm or less.
 6. The improvement of claim 1, wherein the passive metal passive metal electrical interconnection includes a plurality of thin strips.
 7. The improvement of claim 1, wherein the passive metal passive metal electrical interconnection is bare metal or insulated.
 8. The improvement of claim 1, wherein the passive metal passive metal electrical interconnection comprises one or more passive-metal-clad copper wires.
 9. A cathodic protection system, comprising: a structure acting as a cathode to be protected from corrosion; a sacrificial anode in electrical communication with the structure; and passive metal electrical interconnection to the anode, the cathode, or both the anode or cathode.
 10. The cathodic protection system of claim 9, wherein the anode is connected directly to the cathode in a passive “sacrificial anode” configuration.
 11. The cathodic protection system of claim 9, further including an external DC electrical power source to provide current between the anode and cathode in an “impressed current” configuration.
 12. The improvement of claim 9, wherein the passive metal includes titanium, tantalum, niobium, zirconium, nickel, or alloys thereof.
 13. The improvement of claim 9, wherein the passive metal passive metal electrical interconnection is coated with a mixed-metal oxide.
 14. The improvement of claim 9, wherein the passive metal passive metal electrical interconnection includes one or more wires, each with a diameter of 3 mm or less.
 15. The improvement of claim 9, wherein the passive metal passive metal electrical interconnection includes a plurality of wires, each with a diameter of 1 mm or less.
 16. The improvement of claim 9, wherein the passive metal passive metal electrical interconnection includes a plurality of thin strips.
 17. The improvement of claim 9, wherein the passive metal passive metal electrical interconnection is bare metal or insulated.
 18. The improvement of claim 9, wherein the passive metal passive metal electrical interconnection comprises one or more passive-metal-clad copper wires. 